The present invention relates to golf shafts and in particular to a method of filament winding a golf shaft.
Initially wood was the preferred material for making golf shafts. Later steel and then carbon fiber reinforced with an epoxy resin, commonly referred to as graphite, became the preferred material for golf club shafts. Traditionally shafts for golf clubs have been made with a generally circular cross section that gradually tapers downwardly from the upper butt section down to the tip section where it is connected to a club head to form a complete golf club.
Historically all golf club shafts have been designed and produced with an outside diameter at the upper butt end of the shaft, where the hands of the golfer typically grip the golf club, in the range of 0.560 inches to 0.640 inches but have been known to range up to 0.845 inches in diameter. The shafts taper to a lesser diameter at the lower tip section of the shaft in the range of 0.330 inches to 0.370 inches for shafts used with wood or metal wood golf clubs. When used with irons, the tip diameter of the shaft is in the range of 0.335 inches to 0.400 inches.
Variations of golf shafts that do not taper gradually include the use of a wide variety of multiple step-down designs where the decrease in diameter between each step-down section is relatively small. Other shaft designs that alter the gradual tapering geometry of a golf shaft are disclosed in U.S. Pat. Nos. D431274, 5,316,299, 6,454,662, 5,944,618, and 6,827,656 among others.
The most employed manufacturing process for graphite shafts is termed table rolling. Specific graphite/epoxy material commonly known as prepreg, is cut into a specific shape or pattern and each piece, or pattern of prepreg is rolled onto a specifically shaped mandrel using either a flat, dual pattern rolling machine or the prepreg is applied manually. A poly tape is then wound around the prepreg layup that serves to further compact the layers and apply mechanical pressure during the curing process. After the curing hardens the resin, the poly tape is removed and the shafts are cut to length, sanded and the final paint and other cosmetics applied.
When making a shaft with complex geometric shapes and/or diameters, the table rolling method is not applicable since the flat patterns cannot apply sufficient pressure to the non-flat surfaces. The preferred method of making a geometrically different shaft historically has been by molding. These complex geometry shafts are made using a filament winding method that has no limitations of geometry and is much more cost effective than molding. Filament winding differs from table rolling in several different ways.
Specifically, designed mandrels which may be 50 to 70 inches long, are suspended horizontally under tension in a specifically designed winding machine and rotated using a computer program at different speeds to match the profile of the shaft geometry represented by the complexity of the mandrel shape. The graphite winding is fed from a supply such as a spool or supply bundle that typically goes through a resin bath but can also use pre-impregnated tows of fiber. The fibers are attached to a carriage unit that traverses back and forth along the length of the mandrel. Different speeds of both the mandrel rotation and carriage travel speed dictate the angle of the fibers and thus the design of the shaft part being wound. Known methods of manufacturing shafts in this manner use constant mandrel spin speeds and speed of the carriage traversing unit. The cured shafts are then processed similar to a table rolled shaft described above.